1) Field of the Invention
The present invention generally relates to an image formation apparatus wherein an electrostatic latent image formed on an image carrying body such as a photosensitive body, a dielectric body or the like is electrostatically developed with a developer or toner, and the developed image is transferred from the image carrying body to a recording medium such as a sheet of paper.
Also, the present invention is directed to a developing device forming a part of such an image formation apparatus, and further to a conductive rubber roller used in the developing process and/or the transferring process in the image formation apparatus.
2) Description of the Related Art
As a representative of the image formation apparatus as mentioned above, an electrophotographic recording apparatus is well known, wherein the following processes are typically carried out:
a) a uniform distribution of electrical charges is produced on a surface of an electrostatic latent image carrying body; PA1 b) an electrostatic latent image is formed on a charged area of the body surface by an optical writing means such as a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array or the like; PA1 c) the latent image is developed as a visible image with a developer or toner, which is electrically charged to be electrostatically adhered to the latent image zone; PA1 d) the developed and charged toner image is electrostatically transferred from the body to a recording medium such as a sheet of paper; and PA1 e) the transferred toner image is fixed and recorded on the paper. PA1 a) a suitable resin material, e.g., polyurethane, urethane, silicone, or the like, containing a conductive filler such as carbon black or a fine metal powder, and a water-soluble foam-providing substance such as polyvinyl alcohol or methyl cellulose is extruded as a long tubular product; PA1 b) the extruded tubular product is immersed in a body of water held by a container, and thus the water-soluble foam-providing substance dissolves in the body of water so that a foam structure is given the tubular product; PA1 c) the tubular foam product is cut into tubular roller elements having a predetermined length; and PA1 d) the tubular roller element is mounted and fixed on a metal shaft member by using a suitable adhesive such as a thermosetting adhesive, to thereby produce a conductive foam rubber roller.
Typically, the electrostatic latent image carrying body may be an electrophotographic photoreceptor, usually formed as a drum, called a photosensitive drum, having a cylindrical conductive substrate formed of a metal such as aluminum, and a photoconductive insulating film bonded to a cylindrical surface thereof and formed of an organic photoconductor (OPC), a selenium photoconductor or the like.
As one type of developer used in the developing process, a non-magnetic developer is well known, and is composed of only a toner component, i.e., colored fine resin particles. A developing device using the non-magnetic type developer includes a vessel for holding the developer, and a conductive solid rubber roller provided within the vessel as a developing roller in such a manner that a portion of the solid rubber roller is exposed therefrom and is pressed against the photosensitive drum. When the conductive rubber roller is rotated within the vessel in which the developer is held, the toner component is frictionally entrained by the surface of the rubber roller to form a developer layer therearound, whereby the toner component can be brought to the photosensitive drum for the development of an electrostatic latent image formed thereon. The developing device further includes a blade member which is engaged with the surface of the developing roller to uniformly regulate a thickness of the developer layer formed therearound, so that an even development of the latent image can be carried out. The blade member also serves to electrically charge the toner component by a triboelectrification therebetween. In this developing device, the development Is carried out in such a manner that, at the contact area between the photosensitive drum and the rubber roller with the developer layer, the charged toner component is electrostatically attracted and adhered to the latent image due to a bias voltage applied to the rubber roller.
In the above-mentioned developing device for the non-magnetic type developer, the coefficient of surface friction of the rubber roller can be changed by environmental factors, especially, temperature and moisture content. If the friction coefficient of the rubber roller falls, a sufficient amount of the toner component, which is necessary for the development of the latent image, cannot be entrained by the rubber roller.
The Examined Japanese Utility Model Publication (Kokoku) No. 60-6946 discloses a solid rubber developing roller having a rough surface by which the toner particles can be sufficiently entrained though the friction coefficient of the roller becomes small. Nevertheless, when a coefficient of friction of the toner component becomes large due to a rise in temperature and moisture content, the toner component entrained by the rough surface of the rubber roller may be eliminated therefrom by the blade member for regulating the thickness of the developer layer formed on the developing roller, due to an increase of the frictional coefficient of the toner component.
U.S. Pat. No. 5,076,201 discloses a developing roller for the non-magnetic type developer, which is formed of a conductive open-cell foam rubber material so that pore openings appear on the surface of the developing roller. This open-cell foam rubber developing roller is softer than the solid rubber roller, and thus the toner component entrained by the rough surface of the rubber roller cannot be eliminated therefrom by the blade member for regulating the thickness of the developer layer formed on the developing roller, although the frictional coefficient of the toner component is increased due to the rise in temperature and moisture contents.
In the transferring process wherein the developed toner image is electrostatically transferred to a recording medium such as a sheet of paper, a conductive foam rubber roller is also used as an electric charging roller. The conductive foam rubber type charging roller is resiliently pressed against the photosensitive drum, and is connected to a suitable electric source so as to give the sheet of paper an electric charge having a polarity opposite to that of the developed toner image, whereby the developed toner image can be electrostatically transferred to the sheet of paper during a passage of the sheet of paper through a nip between the photosensitive drum and the charging roller.
It is preferable to use the conductive foam rubber roller as the developing roller and the charging roller, because these roller can have a suitable softness so that an operating life of the photosensitive drum can be extended as long as possible. Namely, the harder the developing and charging rollers resiliently pressed against the drum, the greater a wear of the photoconductive insulating film of the drum.
Conventionally, the conductive foam rubber roller may be produced by the following processes:
This conductive foam rubber roller must be further treated before it can be used as a developing roller or a charging roller as mentioned above, because the tubular roller element is covered at an outer surface thereof by a solid skin layer having a thickness of about 1 to 5 .mu.m and the content of the conductive filler is very small in comparison with that of the conductive filler in the internal foam structure of the tubular element. Namely, the conductive foam rubber roller is finished by removing the solid skin layer from the tubular roller element thereof.
Nevertheless, many of the finished conductive foam rubber rollers cannot have a desired electric characteristic when an electrical energy is applied to the tubular roller element through the metal shaft member, due to an existence of an inner solid skin layer formed at an inner wall surface of the central bore of the tubular roller element. In particular, an electric resistivity of the inner solid skin layer is considerably higher than that of the foam structure of the tubular roller element because a content of the conductive filler therein also is very small, and a thickness of the inner solid skin layer is variable along a longitudinal axis of the metal shaft member. Thus, an electric potential of the tubular roller element, which is obtained by the application of the electrical energy to the tubular roller element through the metal shaft member, is also variable along the longitudinal axis of the metal shaft member. Accordingly, for example, when this defective conductive foam rubber roller is used as a developing roller, it is impossible to obtain an even development of the latent image.
Also, U.S. Pat. No. 5,076,201 discloses that the pore opening of the conductive foam rubber developing roller should be at most twice an average diameter of the toner component to prevent a penetration of the toner component into the pore openings thereof. This conductive foam rubber developing roller cannot endure the recording operation in which the number of recorded sheets exceeds, for example, 30,000, because the pore openings thereof are completely blocked due to the penetration of the toner component. Accordingly, the conductive foam rubber developing roller must be frequently exchanged with a new one, for a proper development of an electrostatic latent image to be maintained.
Further, U.S. Pat. No. 5,076,201 further discloses that the blade member for uniformly regulating the thickness of the developer layer formed around the developing roller is formed of a conductive material such as metal, and is subjected to an application of electrical energy to electrically charge the toner component at a given polarity by a charge-injection effect during the regulation of the developer thickness. Nevertheless, a small part of the charged toner component may be reversely charged for the reason stated hereinafter in detail. Of course, the reversely-charged toner component results in a photographic fog.
On the other hand, there is a tendency toward a miniaturization of the image formation apparatus, especially when it is constructed as a personal use type. Accordingly, the developing device also must be miniaturized in proportion to the miniaturization of the image formation apparatus, so that the shaft member of the conductive foam rubber developing roller is obliged to be made slender. In this case, when the blade member is pressed against the developing roller at a given pressure for regulating a thickness of the developer entrained thereby, the slender shaft member of the developing roller is resiliently deformed or bent at a very small degree, so that the developing roller cannot be subjected to a uniform distribution of pressure by the blade member. Namely, a pressure exerted on a central zone of the developing roller is smaller than that exerted on the end zones thereof, and thus a regulation of the developer thickness cannot uniformalized by the blade member. Of course, the non-uniform developer layer results in uneven development of an electric latent image.